This invention relates to modems which use frequency shift keying (FSK) as the modulation method and more specifically relates to the method, and corresponding apparatus, of demodulating data transmitted using FSK modulation.
Type 103 modems use FSK modulation in which two different frequency tones represent the two data states. Two pairs of tones are utilized depending upon whether the modem was the originating or answering modem. Phase locked loops and zero crossing detectors have been utilized as FSK demodulators. Both of these types of demodulation techniques require precise phase information of the two signalling tones utilized.